Direct evidence of the gradient drift instability being the origin of a rotating spoke in a crossed field plasma

TL;DR

This study provides direct evidence that the gradient drift instability is the fundamental cause of rotating spokes in crossed field plasmas, using comprehensive kinetic simulations and theoretical analysis to identify the instability modes involved.

Contribution

The paper demonstrates, through detailed simulations and theory, that the gradient drift instability underpins the formation of rotating spokes in crossed field discharges, confirming its dominant role.

Findings

The spectrum of gradient drift instability modes was fully characterized.

The lower hybrid mode was identified as the most unstable linear mode.

The transition from linear instability to nonlinear spoke formation was observed.

Abstract

A plasma rotating spoke in a crossed field discharge is studied using 2D radial-azimuthal fully kinetic Particle-In-Cell Monte Carlo Collision (PIC/MCC) simulations. The kinetic model reveals the whole perturbation spectrum of the gradient drift instability in the linear stage: Simon-Hoh, lower-hybrid and ion sound modes, providing direct evidence of the spoke of the gradient drift instability nature. The two-fluid dispersion relation of the gradient drift instability was utilized to analyze the linear development of instabilities in the simulations. The charge separation effect was incorporated in the fluid linear theory and a super-resolution signal processing method (multiple signal classification) was applied to obtain the numerical frequency spectrum. The simulated spectrum and growth rate show excellent agreement with the theoretical dispersion relation (real frequency and imaginary frequency) over investigated cases. The most linearly unstable mode was found to be the lower hybrid instability and the mode transition into the m=1 macroscopic rotating structure after saturation of the linear phase is accompanied by an inverse energy cascade. In the nonlinear stage, the pronounced spoke phenomena can occur when the heating of $\mathbf{E_{\theta}\times B}$ electron flow channeled in the spoke front passage suffices to provide the enhanced ionization.